It is known within the oil- and gas-drilling industry to take measures for the circulation of drilling fluid to be maintained in the borehole while a drill string is being extended. NO 326427 discloses a system in which a top-drive drilling machine with a hollow drive shaft cooperates with a sluice chamber provided with seals surrounding the pipe, the drilling fluid being directed alternately through the drive shaft via a first fluid inlet, when a continuous drill string is connected to the drilling machine, and through the sluice chamber via a second fluid inlet, when the upper end of the pipe string is arranged in the sluice chamber and is going to be connected to a new pipe section.
A drawback of the prior art according to NO 326427 is that the pipe-string rotation ceases when the pipe string is to be extended. It is known within the industry that it is an advantage to maintain the pipe-string rotation both in order to reduce the risk of the pipe string sticking and in order to improve the productivity, for example increase the drilling capacity by not having a stop in the actual drilling operation while the drill string is being extended.
From NO 20100123 an arrangement is known in which, between a first top-drive drilling machine and a borehole, a second drilling machine is arranged, which is provided with a rotary table arranged to take the weight of a pipe string, a rotary-drive unit arranged to continuously rotate the pipe string, and a fluid chamber which is arranged to connect a pipe-string end portion to a drilling-fluid system in a fluid-communicating manner, the fluid chamber being provided with pipe-string ports including means arranged to fluid-sealingly close the pipe-string ports, and the second drilling machine further being provided with a power tong which is arranged to connect/release an element to/from the pipe string, the power tong being arranged in the fluid chamber. The drawback of this arrangement is that the rotary-drive unit is directly connected to the fluid chamber (sluice chamber) and the power tong is enclosed in the fluid chamber. Here, an adjustment of the power tong to the relevant pipe dimension that is being handled must be made by intervention in the fluid chamber.